The present invention relates generally to cordless telephony. More particularly, the invention relates to improved methods and apparatus for measuring the performance of an analog cordless telephone set.
Cordless telephones are widely used and provide great convenience to their users. A cordless telephone provides many of the advantages of a wireless telephone, typically at a much lower cost. A cordless telephone does not provide the great mobility of a wireless telephone, but a cordless telephone user is able to use the telephone handset at some distance from the base station, and can do this simply by using his or her ordinary telephone service, which typically allows unlimited local calling for a low flat rate.
Performance of cordless telephones is affected by a number of conditions, including distance between the handset and the base station, RF conditions in the local area where the telephone is operating, and the like. It is of considerable benefit to cordless telephone users to have cordless telephones possessing signal strength performance testing characteristics. Performance information about digital telephones is often obtained by monitoring the bit error rate. Digital telephones transmit speech and other sounds by representations of digital bits. The detection of the proper arrival of bits or the failure of bits to arrive properly and the computation of a bit error rate representing the proportion of bits which fail to arrive are performed as part of the normal operation of some digital telephones and can be employed to provide performance information about these digital telephones. Transmission of speech and other sounds by means of bits and monitoring of bit error rates is not possible in analog cordless telephony and so obtaining performance information in this way is not available to users of present day analog cordless telephones.
It is possible to test the performance of an analog telephone by transmitting a test signal from the handset to the base or from the base to the handset and then using measuring instruments to monitor selected characteristics of received signal, for example, amplitude, frequency, distortion, signal to noise ratio and the like. Accomplishing this, however, requires that measuring instruments or laboratory test equipment be used. For this reason, it is difficult to provide an analog cordless telephone which is equipped to perform range testing without a need for separate instruments to measure signal quality. Instruments for measuring analog signal characteristics such as amplitude, frequency, distortion and signal to noise ratio are typically laboratory instruments and cannot feasibly be employed as functional components of analog cordless telephones. Their inclusion in a telephone would lead to a telephone having features which added greatly to cost, weight and complexity, but were not useful in the normal functioning of the telephone.
The range of the telephone is the distance at which the handset can communicate effectively with the base station, that is, at which a signal transmitted by the handset can be reliably received at the base station or a signal transmitted by the base station can be reliably received at the handset. The range of a cordless telephone is of great interest to most users. Users normally want cordless telephone systems with as great a range as possible, and they also want to know the range of the system they have or are contemplating purchasing. In prior art analog cordless telephones, range testing is accomplished by having two people go to a suitable site. One person takes the base station, while the other person takes the handset. The distance between the handset and the base station is varied, typically by having the person with the handset walk away from the base station, and performance is monitored. The range of the system is determined to be the distance at which performance becomes unacceptable.
The drawbacks of this method of range testing are obvious. First, it is inconvenient for persons at a manufacturer or repair facility to move a base station and a handset away from each other while monitoring performance in order to determine the range. Second, unless the testing personnel have some way of knowing how far away they are from one another, the range determination will only be an estimate, and may be quite inaccurate. Third, the results of such a test are not repeatable unless the test range is free of RF environmental obstructions. A digital telephone can be subjected to automatic range testing by having the handset communicate with the base station, or vice versa, and determining the bit error rate under whatever range and RF conditions are present. As noted above, the bit error rate is not available for evaluation in an analog telephone.
An analog telephone can be subjected to automatic range testing by placing the handset and base station in shielded boxes to remove the effects of environmental obstructions and other factors affecting the RF signal and allowing the base and handset to transmit and receive a test signal through sensing antennas connected via an attenuator. The signal characteristics are monitored by measuring equipment and the signal characteristics and the attenuation are employed to perform range calculations. Measuring equipment is not typically included in an analog telephone because it is not used in the normal operation of the telephone. The need for measuring equipment therefore adds to the complexity of the test system.
There exists, therefore, a need in the art for techniques for testing the performance of an analog cordless telephone which employ features used in the normal operation of the telephone to provide performance information.
Two different characteristics govern the quality of a link between the handset and base of a cordless telephone. The first characteristic is voice quality. The second characteristic is the reliability of signaling between the handset and base, that is, from the handset to the base or vice versa. Signaling is the sending of commands between the handset and the base, for example when the handset tells the base to go off hook or when the base notifies the handset of an incoming call and directs the handset to ring. Signaling in analog cordless telephones is carried out by the transmission and reception of operational code (opcode) packets which are transmitted and received by the handset and the base station as part of their normal functioning. Typically, a cordless telephone link exhibiting good signaling characteristics also exhibits good voice characteristics under the same conditions, but it is not necessarily the case that a cordless telephone link exhibiting good voice characteristics will also necessarily exhibit good signaling characteristics. This is true because the allowable FM deviation is greater for data packets than for analog voice signals. Tuning imperfections in radio receivers typically degrade reception of signals with larger FM deviation more than signals with lower FM deviation. For this reason, data packet performance provides a useful measure of both data and voice performance but voice performance does not provide a good measure of data packet performance.
One of the most important factors affecting the quality of a link between a handset and a base station is the distance between the handset and the base station, and an important characteristic of the performance of a handset and base station combination is its range, that is, the distance over which it is able to maintain a link of acceptable quality. Because the quality of a link between a base station and a handset is usually dependent on the signaling characteristics exhibited by the link, the maximum range of an analog cordless telephone, is usually dependent on the signaling range of the telephone.
A system for testing an analog cordless telephone according to one aspect of the present invention employs the creation and transmission of test packets, which are similar to opcode packets normally employed in transferring commands and status information between the base and the handset. If it is desired to test the communication performance between the handset and the base station, the handset sends a plurality of test packets to the base station, and the number of packets received is compared against the number expected to produce a handset to base station error rate characterizing the quality of the link between the handset and the base station in conveying signals transmitted from the handset to the base station. The handset to base station error rate can be compared against a predetermined criterion indicating acceptable or unacceptable performance. Similarly, if it is desired to test communication performance between the base station and the handset, the base station may send a plurality of test packets to the handset, and the number of packets received may be compared against the number expected to produce a base station to handset performance error rate. The base station to handset error rate is evaluated to determine the quality of the link between the base station and the handset. By implementing appropriate software in the telephone, it is possible to produce a numerical performance measure on an open range or in a customer""s home or office environment by properly evaluating the handset to base station error rate, the base station to handset error rate or both.
When more precise testing of telephones is needed in a repair or manufacturing environment, appropriately designed cordless telephones can use these test packets to make range measurements. A cordless telephone may suitably be designed to allow control of the telephone by and reporting to a remote computer through a wired connection to a test access port on the handset and another test access port on the base station. The telephone is placed in a test environment exhibiting repeatable conditions, especially with respect to RF interference and obstructions. Such an environment may by achieved by providing a first shielded box for containing the handset and a second shielded box for containing the base station. A first sensing antenna is present in the first shielded box and a second sensing antenna is present in the second shielded box. The first and second sensing antennas are connected via an attenuator. The amount of attenuation introduced by the attenuator is calibrated to be equivalent to a specified distance.
A computer such as a personal computer (PC) is connected to the handset and the base station by a test access port on each of the handset and the base station. The test controller directs the handset to transmit a specified number of range test packets to the base station. The packets are transmitted to the first sensing antenna, attenuated, conveyed to the second sensing antenna and transmitted to the base station. The base station informs the test controller of every packet received. The test controller computes a packet error rate based on a difference between the number of packets received and the number of packets expected. The test controller uses this rate, along with the calibrated attenuation, to determine the range performance from the handset to the base. A similar test may be performed by having the base station transmit packets to the handset, with the handset informing the test controller of every packet received. The test controller then uses the packet error rate to determine range performance from the base station to the handset. It is also possible to perform iterative testing, in which a series of tests is performed with attenuation increased between tests, until the error rate exceeds some predetermined criterion.
A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following Detailed Description and the accompanying drawings.